Fig. 1: A modest sample of native arsenic. The mineral is notably dense, specific gravity 5.7, which is immediately noticeable on picking up the specimen. A fracture in the arsenic seems to have channeled subsequent alteration, forming the sulphide realgar (AsS), a flamboyantly orange-red mineral that is often the best visible indicator of the presence of large amounts of arsenic. Sample 1820 from David Shannon Minerals, 1997, weight 150.53 grams, 4.5 x 4 x 3 cm.
Note that the proper mineral name is arsenic, but I use native arsenic where confusion can occur between the mineralogical and chemical contexts, the latter being the behaviour of arsenic as a chemical element, whether pure (As), or in compound form with one or more other elements (e.g., AsS) or dissolved, in solid solution, as a lesser component of another mineral (e.g., ZnS).
"Rock of the Month # 268, posted for October 2023" ---
Arsenic is one of some 33 elements in the periodic table that occur as native elements, i.e., largely uncombined with other elements (such as oxygen or sulphur) and occurring in their own particular crystalline form (even if that is invisible to the naked eye). Some native elements are much better-known and more widely-distributed, such as gold, silver and copper, or carbon in its very different guises as diamond and graphite (see Wilson in Boyle, 2024, Text Box 1 and Chapter 3C endnotes). Some are of very restricted occurrence, such as native iron (conditions on Earth are such that iron usually occurs as oxides, carbonates, silicates and other compounds, in conjunction with other elements). Still others are vanishingly rare. Native arsenic is relatively common, though the element is found more often in sulphides and sulphosalts and other compounds. The MINLIB bibliographic database has some 3,200 records (3.5% of the total) that are flagged for arsenic, but just 86 that mention native arsenic. The element demands attention around the world, from China and India to Cornwall and Nevada. A myriad assemblage of secondary minerals of arsenic, such as oxygen-bearing arsenates and phosphates, can form in terrestrial weathering (Bowell et al., 2014).
Arsenic is commonly found at lower concentrations, not only in those sulphides and sulphosalts which contain essential arsenic, but as a minor component of ore minerals of other elements. Minerals that may incorporate ppm to percent levels of arsenic include oxides (cassiterite), sulphides (pyrite, sphalerite) and native copper. Thus arsenic occurs in the ore deposits of other elements, notably gold and silver as well as base metals.
Native arsenic has trigonal symmetry, and also has two rare orthorhombic trimorphs, arsenolamprite and pararsenolamprite. Apparently white when freshly polished, it tarnishes rapidly to grey or black, with a submetallic to dull lustre. The dull, dark grey surface is due to oxidation to arsenolite, naturally-occurring As2O3. A fresh face in reflected light is white with a creamy tint, not as bright as silver. It is anisotropic, may show twin lamellae, and often displays a colloform texture of concentric layers, or radiating spheroids (Uytenbogaardt and Burke, 1971, pp.84-85). The element arsenic has atomic number 33, and like aluminium and gold has just one stable isotope: 75As. The estimated average crustal abundance of arsenic is 2.5 ppm, which is roughly comparable to tin, say, and about one-tenth as abundant as copper.
The two small samples shown here are both from the Kuse mine, near the gold-mining town of Bau, southwest of the regional capital Kuching in the westernmost part of Sarawak (a part of Malaysia, on the northern coast of Borneo).
THE BAU DISTRICT, AND A FEW OTHER ARSENIC LOCALITIES
Wilford (1955) wrote up his surveys of what was then British Borneo, 1949-1955. The region has a hot and humid tropical climate and a varied topography, which is a reflection of diverse rock types. The oldest rocks are schists of possible Devonian age. Economic assets include gold, arsenic and antimony, as well as diamonds. Jurassic and Cretaceous limestones at Bau are cut by a line of Tertiary dacite and microgranodiorite intrusions. The main Au deposits occur in limestone and immediately overlying shale near the intrusions. Ore minerals include sphalerite and galena, native arsenic and native antimony. Percival et al. (1989) noted that the Au deposits of Bau have yielded 1.2 million ounces of Au plus significant amounts of Sb and Hg. There is an inferred association between calc-alkaline intrusions, vein deposits, and replacement-type disseminated Au deposits. Hydrothermal alteration and Au mineralization occur in massive limestone and shale in the core of the Bau anticline, whose crest is cut by several sets of near-vertical faults. There is a structural control of the Miocene granodiorite and dacite porphyry intrusives. Silica, native arsenic, stibnite, pyrite, realgar and local arsenopyrite were all deposited from fluids into brecciated shale and limestone along faults and limestone-shale contacts. The resulting jasperoid and replacement bodies account for most of the local Au production. Some ores carry >20% As.
Mustard (1997) notes evidence of old mining activities across a 250 km2 area around the township of Bau. Chinese miners were active in the district as early as 1820. Mustard cites cumulative production, by 1997, as >2 million oz from placer Au occurrences and over 50 hard-rock mines. The Tai Parit mine produced 700,000 oz in 1898-1921 and 1991-1997, and modern resources of about 1 million oz were delineated in the area, mostly in the Jugan and Pejiru deposits. Triassic to Jurassic andesitic volcanics are the oldest rocks that outcrop within the Bau Au field. This igneous basement is overlain unconformably by 500 m of late Jurassic massive micritic limestone, in turn followed by a 1,500-m-thick flysch sequence (mostly shale). A NNE-striking belt of Miocene intrusives (granodiorite porphyry stocks and dykes) cuts the field, and all known deposits lie within 5 km of this. The Bau field is complex with some seven categories of mineral deposits. There are some Hg-As-Ba deposits. The dissolution of limestone and karst / collapse breccia formation is important in formation of some of the sediment-hosted deposits. Gold occurs in silica and in arsenical pyrite (according to unpublished data, pyrite at Pejiru contains up to 18% As, surely a record). Hutchison (2005) reviewed the whole northern fringe of the great island of Borneo. He notes the "non-volcanic epithermal deposits" of the Bau district (ibid., pp.151-156). High-level granite, granodiorite and diorite plutons provided a heat source to remobilize metals from older continental crust. Volcanism is lacking, and there is often an important Hg association. Western Sarawak is marked by important Miocene epizonal magmatism, age dated at 23 to 8 Ma. As noted by earlier workers, Hutchison explains that the Bau mining district is related to that line of small, high-level Tertiary dacitic to granodioritic stocks and dykes. The complex arsenical ores with native arsenic, stibnite, pyrite and native gold appear, based on fluid inclusions, to have formed at low temperatures (140-250°C). The mineralization may be akin to Carlin (Nevada, USA) and Bingham (Utah, USA). The lower temperatures are appropriate to an epithermal regime where native arsenic, realgar, cinnabar and other minerals occur. Concerning the arsenical Au ores of the Krokong area of Bau, a representative bulk analysis of such ore is said to grade 12 ppm Au, 1.68 ppm Ag, a remarkable 8.09% As and 1.91% Sb. Not surprisingly, arsenic and cyanidation (in old workings) represent an unsolved environmental problem (ibid., p.155).
Europe has a number of localities for native arsenic, which occurs, e.g., in brecciated marbles in eastern Carinthia, in Austria (God and Zemann, 2000). Lengenbach in the Valais of Switzerland is a famous locality (Roth et al., 2014). This site is roughly 65 km E.N.E. of Sion near the Swiss-Italian border. As of December 2013, Lengenbach was the type locality of a remarkable 31 mineral species. There are more than 125 known species in this dolomite-hosted deposit, including many arsenic sulphides and sulphosalts, as well as native arsenic. Johan (1985) describes the Cerny Dul deposit in the Giant Mountains of the Czech Republic. This is a different style of deposit, with calcite veins in metamorphic rocks, with a complex paragenesis of various arsenides and native arsenic. Hydrothermal fluids with neutral to acid pH may be able to carry significant arsenic, which can thus occur in significant quantities in tin deposits containing cassiterite and arsenopyrite, such as: Llallagua in Bolivia; Panasquiera in Portugal; and Cligga Head and Geevor, Cornwall, S.W.England (Heinrich and Eadington, 1986). In Cornwall, the Dolcoath mine in Camborne was notable for being profitable for both copper and tin, with arsenic as a byproduct (Embrey and Symes, 1987). Arsenic is found in the hotter vein systems, including those with greisen alteration, as at Cligga Head.
Canada has many documented occurrences of arsenic mineralization (Hurst, 1927), from the Yukon to Newfoundland. Papezik (1966, 1967) described an occurrence of native arsenic in the Whalesback and Little Bay Cu mines on the Springdale Peninsula, Newfoundland. The largest mass filled a fracture in an altered basic dyke on the 1500 level of the Little Bay mine. The arsenic displayed a rough colloform texture and contained inclusions of rammelsbergite (NiAs2). The arsenic may have formed by decomposition of arsenopyrite -rich sulphides intruded by the dyke.
Fig. 2: Another sample, displaying the rounded, onion-skin shells and rounded (reniform, or kidney-shaped) surfaces that typify better-known minerals, including the "kidney ore" variant of the iron oxide hematite (Fe2O3). As well as orange realgar, traces of other minerals may occur, such as silvery prisms of the common antimony sulphide stibnite (Sb2S3) or the rare stibarsen (stibarsenic, SbAs). Sample 1662 from David Shannon Minerals, 1995, weight 52.95 grams, 3.3 x 2.8 x 2.4 cm.
ARSENIC in the ENVIRONMENT
Arsenic, like antimony, is one of a number of "indicator elements" used in geochemical exploration, as a vector towards deposits of gold especially, where these elements may also be concentrated. The assay reveals the element, even in the absence of some of the colourful secondary minerals found in the weathering zone, such as realgar and orpiment, annabergite and erythrite. The details of the use of arsenic as an "exploration vector", and the toxicology of arsenic affecting human and animal health, are beyond the scope of this short(-ish) essay. Arsenic occurs in the environment, generally in the form of sulphides, and commonly as a minor to trace component (mostly <1 wt.% down to parts per million or below) of common sulphides such as pyrite (FeS2 - fool's gold).
Arsenian or arsenical pyrite with percent levels of arsenic is generally associated with ore deposits, especially of gold, but lower levels occur also in pyrite in widespread sedimentary strata. This can cause regional health effects, as in areas of the northern Indian subcontinent, where wells penetrating pyritic sediments promote oxidation, allowing arsenic to enter the well water to be ingested by the human and livestock populations on the surface. The situation in Bangladesh and adjacent West Bengal became critical, and was intensively studied in the 1990s. Endemic arsenic poisoning occurred via tube well waters in West Bengal, with deep wells emitting waters with arsenic levels to 3.7 ppm, way over the WHO permissible limit of 0.05 ppm As for drinking water (Banerjee, 1993). Detailed findings of a survey in a region of West Bengal (Mandal et al., 1996) identified 560 villages that were affected, with over 1 million people drinking arsenic-contaminated water and over 200,000 having arsenic-related diseases. Analysis of about 20,000 water samples from tube wells indicated that 45% had arsenic content >0.05 mg/L (50 ppb, 0.05 ppm).
Williams (2001) reviewed arsenic measurements in the mine waters of 34 gold and base-metal mining localities in seven countries, including in Sarawak, Brazil and Ecuador. The peak values at these sites range from 0.005 to 72 mg/l, with the potable water threshold (as noted above) exceeded in 25 cases. Arsenic-rich materials are a generally-unwanted byproduct of the processing of ores from a range of mineral deposit types.
As a case study of arsenic as a byproduct, and the problems which that poses, let's consider the occurrence of arsenic at the old Giant gold mine, which is located near Yellowknife, the capital of the Northwest Territories of Canada. Here gold deposits were hosted in large part along shear zones cutting metavolcanic rocks ("greenstones"), with concentrations of quartz, carbonates and sulphides, and elevated levels of arsenic, antimony, gold, silver and other elements (Boyle, 1961). The Giant mine lived up to its name, working a world-class gold deposit that produced more than 7 million ounces (220 tonnes) of gold metal. Jamieson (2014) studied the situation and noted that, from 1949 to 1999, ore was roasted as pretreatment for cyanidation, a common treatment regime in cases where much of the gold is locked into "refractory" sulphides such as arsenopyrite. Arsenopyrite (FeAsS) in the ore accounted for an estimated 20,000 T of As2O3 released via stack emissions, a dramatic point source of arsenic pollution. Over the mine life, an estimated 237,000 T As2O3 was stored underground as the mining proceeded. This was the residue from working the ores from the closed Giant and Con mines. The waste is stored in 14 large underground vaults (Braden, 2010). Arsenical maghemite (dimorph of hematite) and hematite were also produced, and, in mine tailings, these now leach As into the environment. Biofilms in the old mine workings include yukonite, an hydrated Ca Fe arsenate (Jamieson, 2014). The mine tailings contain fine-grained sulphides such as pyrite and arsenopyrite. The calcine residue is perhaps 10% of the tailings by weight, but contains 80% of the total As load. Iron oxide products of the roasting, such as nanocrystalline maghemite, contain up to 7 wt.% As (Walker et al., 2015).
ARSENIC in the ECONOMY
Arsenic has a range of traditional uses, in pesticides and wood preservatives, and other applications in glass manufacture, lasers, and even in medical procedures. More recently came the realisation that gallium arsenide (GaAs) might yield superior semiconductors. However, the uptake seems to have been slow relative to the standard silicon chips. If widely adopted, this would boost the market for gallium metal, and provide a valuable use for some of the surplus arsenic! Electron velocities are faster in GaAs than Si, and so GaAs circuits are faster (at equal or lower power) than silicon circuits (Brodsky, 1990). 5G technology for wireless communications should require large amounts of uncommon elements, such as Ga. China contributed 95% of primary Ga production in 2018 (354 out of 372 tonnes), the Ga coming in large part as a byproduct of treatment of bauxite (Al) and Zn ores (Heckbert, 2019). China is also the dominant producer of arsenic at present. I wonder if the Yellowknife "arsenic stockpile" is a possible substitute for primary As production, turning a liability into an asset (?).
Banerjee,R (1993) Death by slow poisoning. India Today 18 no.13, 141-142, July.
Bowell,RJ, Alpers,CN, Jamieson,HE, Nordstrom,DK and Majzlan,J (editors) (2014) Arsenic: Environmental Geochemistry, Mineralogy, and Microbiology. Mineral.Soc.America Reviews in Mineralogy and Geochemistry 79, xv+635pp.
Boyle,RW (1961) The Geology, Geochemistry, and Origin of the Gold Deposits of the Yellowknife District. GSC Memoir 310, 193pp.
Boyle,RW (2024) A History of Geochemistry and Cosmochemistry. Volume 1. Prehistory to the end of the Classical Period (A.D. 476). Cambridge Scholars Press, Newcastle upon Tyne, England (Wilson,GC, Butt,CR and Garrett,RG, editors), in press.
Braden,B (2010) Fixing a Giant mess. Can.Min.J. 131 no.2, 10-13, February.
Brodsky,MH (1990) Progress in gallium arsenide semiconductors. Scientific American 262 no.2, 68-75, February.
Embrey,PG and Symes,RF (1987) Minerals of Cornwall and Devon. British Museum (Natural History) / Mineralogical Record Inc., 154pp.
God,R and Zemann,J (2000) Native arsenic-realgar mineralization in marbles from Saualpe, Carinthia, Austria. Mineralogy and Petrology 70, 37-53.
Heckbert,D (2019) 5G technology should boost demand for gallium, cobalt. Northern Miner 105 no.26, 8, 23 December.
Heinrich,CA and Eadington,PJ (1986) Thermodynamic predictions of the hydrothermal chemistry of arsenic, and their significance for the paragenetic sequence of some cassiterite- arsenopyrite-base metal sulfide deposits. Econ.Geol. 81, 511-529.
Hurst,ME (1927) Arsenic-bearing Deposits in Canada. GSC Econ.Geol.Ser. 4, 181pp.
Hutchison,CS (2005) Geology of North-West Borneo: Sarawak, Brunei and Sabah. Elsevier, 421pp.
Jamieson,HE (2014) The legacy of arsenic contamination from mining and processing refractory gold ore at Giant mine, Yellowknife, Northwest Territories, Canada. In `Arsenic: Environmental Geochemistry, Mineralogy, and Microbiology' (Bowell,RJ, Alpers,CN, Jamieson,HE, Nordstrom,DK and Majzlan,J, editors), Mineral.Soc.America Reviews in Mineralogy and Geochemistry 79, xv+635pp., 533-551.
Johan,Z (1985) The Cerny Dul deposit (Czechoslovakia): an example of Ni-, Fe-, Ag-, Cu-arsenide mineralization with extremely high activity of arsenic: new data on paxite, novakite and kutinaite. TMPM 34, 167-182.
Mandal,BK plus 12 (1996) Arsenic in groundwater in seven districts of West Bengal, India - the biggest arsenic calamity in the world. Current Science 70 no.11, 976-986, 10 June.
Mustard,H (1997) The Bau gold district - east Malaysia. In `World Gold '97', Australasian Institute of Mining and Metallurgy Publ. 2/97, 294pp., 67-77.
Papezik,VS (1966) Native arsenic in Newfoundland. Can.Mineral. 8, 670-671.
Papezik,VS (1967) Native arsenic in Newfoundland. Can.Mineral. 9, 101-108.
Percival,TJ, Radtke,AS, Bagby,WC, Gibson,PC and Noble,DC (1989) Bau, East Malaysia: arsenic-rich sedimentary-rock hosted gold deposits spatially and genetically associated with epizonal magmatism. GSA Abs.w.Progs. 21 no.6, Annual Meeting (St. Louis), 294.
Roth,P, Raber,T, Drechsler,E and Cannon,R (2014) The Lengenbach quarry, Binn valley, Switzerland. Mineralogical Record 45, 157-196.
Uytenbogaardt,W and Burke,EAJ (1971) Tables for Microscopic Identification of Ore Minerals. Elsevier, 2nd revised edition, 430pp.
Walker,SR, Jamieson,HE, Lanzirotti,A, Hall,GEM and Peterson,RC (2015) The effect of ore roasting on arsenic oxidation state and solid phase speciation in gold mine tailings. Geochemistry: Exploration, Environment, Analysis 15, 273-291.
Wilford,GE (1955) The Geology and Mineral Resources of the Kuching-Lundu area, West Sarawak, including the Bau Mining District. Geological Survey Department, British Territories in Borneo, Memoir 3, 254pp. plus 2 maps.
Williams,M (2001) Arsenic in mine waters: an international study. Environmental Geology 40, 267-278.
Graham Wilson, 17-23 September 2023
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